Vehicles that are driven entirely or in part by electric motors are becoming more and more common. These motors typically draw power from a high voltage battery pack in the vehicle, through a high voltage DC bus. Electric vehicles with Permanent Magnet motors can experience over-voltage faults across the high voltage DC bus. For example, when the motor is driven by the inertia of the vehicle, or when the vehicle is travelling down a hill, the permanent magnet motor will function as a generator. Under normal circumstances, the energy generated from the motor in these situations is used to charge the traction battery. However, if the main contactor between the battery and the drive inverter is open as a result of a fault condition, the motor may charge the high voltage DC bus to a voltage level that could damage certain components connected to the bus. Some components, such as the high-voltage DC capacitor, which is provided on some electric vehicles, could catch fire in such a situation.
To prevent this problem, an approach is to short circuit the terminals of the motor together, known as a 3-phase short. This is carried out using the 12V power supply from the 12V battery in the vehicle. In a situation, however, where 12V power was not available, (e.g. if there is a failure in the 12V power supply), then the 3-phase short function will not be available if needed.
It would be advantageous to provide a way of keeping the 3-phase short function available in a situation where there is a failure in the 12V power supply.